Often industrial solar cell, for example silicon solar cells, process flows include front surface texturization to reduce optical reflection loss in crystalline silicon solar cells. The front surface, or in other words cell surface facing solar radiation or sunnyside or frontside, is textured to reduce optical reflection losses and to increase the overall absorption of solar radiation. For example, the front surface texturization process may consist of etching the silicon surface in either an alkaline bath containing KOH or NaOH (for monocrystalline silicon cells) or in an acidic bath containing HF and HNO3 (for multi-crystalline silicon cells). The chemical etching of the silicon wafer in the alkaline bath is orientation dependent so that the slowest etching planes (111) are exposed. The etching results in the formation a randomized surface texture of square base pyramids with random micron-scale sizes and orientation that cause the reflected rays to strike the adjacent tilted random pyramid silicon surface thereby increasing light absorption. The sun rays incident on the inclined surfaces of these pyramids are reflected at an angle that increases the probability of their hitting the adjacent surface thereby increasing their absorption in the silicon bulk (and reducing the optical reflection losses of the solar cell). Combined with the use of an anti-reflection coating, such as a thin silicon nitride layer, this may be a very effective method to reduce the optical reflection losses of solar radiation in the solar cell.
However, this wet etching texturization process has a number of disadvantages. First, the amount of silicon material removed using wet alkaline chemistry texturing may be on the order of 5 to 10 microns. For thin film crystalline silicon solar cells where the starting silicon substrate thickness may be in the range of a few microns to 10's of microns, this amount of silicon loss during texturing is clearly excessive and undesirable. Additionally, while this method may be suitable in some instances for monocrystalline silicon solar cells, it has limitations when applied to multicrystalline wafers where the surface orientation of different grains is different.
Further, this type of silicon surface texturing increases surface area with numerous sharp pyramid tips, and may also increase the surface recombination velocity (i.e., the front surface recombination velocity FSRV) of minority carriers and reduce the cell open-circuit voltage. This process may not suitable for the back surface of either a front contact front junction (FCFJ) solar cell or a back contact back junction (BCBJ) solar cell. For front contact front junction solar cell (FCFJ), blanket aluminum is often deposited on the back side (usually as a screen-printed aluminum paste) and annealed or fired to form the aluminum back surface field (BSF) during which the surface texture is destroyed. High efficiency front contact front junction (FCFJ) solar cells have structures where a passivating dielectric film is first deposited on the back side and the blanket film of aluminum makes contact to silicon through contact holes made in the dielectric. Such holes may be formed using pulsed laser ablation. Alternatively, the process of forming localized contacts on the solar cell backside may be performed using a Laser-Fired Contact (LFC) process using a pulsed nanosecond laser tool. The ablation of the dielectric (or formation of LFC contacts) is severely and detrimentally impacted by the presence of the texture. For back contact, back junction (BCBJ) solar cells, the processes that are used to form the structure on the back side are unfavorably affected by the presence of texture on the silicon surface. For these reasons, back surface of high efficiency cells are typically flat (i.e., the back surface of silicon is usually not textured) and the structure consists of a passivating layer of an oxide (or nitride) on this flat surface followed by forming base openings (front contact cells) or both emitter and base openings (back contact, back junction cells) followed by the deposition of a metal film, typically aluminum, that covers the backside surface (the BSF for front contact cells or a large amount of metal surface area for BCBJ cells). Optimization of the oxide (or nitride) film thickness and the reflectivity of the metal film may yield high reflection of sun rays from the back surface. However, this reflection is mostly or completely specular and the sun rays reflected vertically back up from the flat ‘mirror’ have a high chance of escaping the silicon film without absorption. A specular rear mirror reflection does not maximize the mean travel path length of infrared photons, thus the—quantum efficiency for these photons is not improved significantly.